1. Field of Invention
This invention relates generally to measuring current flows, and more particularly, to measuring the current flows in a mechanically fastened composite structural joint.
2. Background of the Invention
Traditionally aircraft have been built using aluminum as the primary airframe structural material as aluminum is an excellent electrical and thermal conductor and provides inherent lightning protection. However, structures (may also be referred to as “parts” or “components”) for aircraft are increasingly being made of carbon fiber composite materials as composites have higher specific strength and stiffness. Using composite materials for fuel filled (a.k.a., wet) composite primary structures poses a significant lightning strike concern as composite structures have a diminished capacity for carrying current as compared to aluminum and are far more susceptible to damage from lightning strike attachments.
When lightning strikes a composite structure on an aircraft, it can cause a spark to occur by attaching to the composite structure such that a high amount of current traverses between various materials in the composite structure and jumps between the dielectric gaps between the various materials. As the sparking threshold for wing fuel tank composite structures vary, it is essential to understand how high-amperage currents can flow through the composite structures of the aircraft.
The risk from uncontrolled transport of current is that certain geometries are prone to developing sparks above geometry-specific threshold levels (threshold is defined as the lowest value of the peak current of a transient lightning pulse at which arcing or sparking is seen to occur). For example, one such geometry involves mechanically fastened skin-substructure joints where exposed fastener heads can conduct high currents from the aircraft exterior into metal substructures. For these geometries, the sparking threshold can be quite low (on the order of 5000 amps), due mainly to the interface between fastener collars/nuts and metal surfaces.
Directly measuring the current in fasteners has been difficult due to space limitations within a densely-fastened joint. Conventional methods for current flow measurement, such as Rogowski Coils, have been impracticable to use because of the space limitations. Rogowski coils are used for current measurement because of the large amount of electromagnetic interference (EMI) noise that is commensurate with extreme changes in current flow due to the arcing/sparking of a composite structure. Rogowski coils are immune to EMI, have a broad frequency response and are linear over a large range of current amplitudes making their use more convenient.
Aircraft manufacturers typically have to certify an aircraft against lightning strikes. How current is flowing through structures in the aircraft is an important element for such certification. However, a problem exists in that it is extremely difficult to measure current flow through individual components in a multi-component structural composite joint.
In view of the above, what is needed is a method and system for measuring current flows through individual components in a multi-component structural composite joint.